1. Field of the Invention
The present invention relates to a process for producing a composite material having high mechanical strength and heat resistance. More particularly, it relates to a process for producing economically and efficiently a composite material composed of a polymer such as polyamide and a layered silicate constituting a clay mineral which are bonded to each other through ionic bonding and are uniformly mixed with each other.
2. Description of the Related Art
Various attempts have been made so far to incorporate an organic polymeric material with an inorganic material such as calcium carbonate, clay mineral, and mica for the improvement of its mechanical properties. One of such attempts has lead the present inventors to findings that the polymerization of polyamide monomer in the presence of a clay mineral forms a composite material having good mechanical strength and heat resistance through intimate mixing at the molecular level of the polyamide and the layered silicate constituting the clay mineral. (See Japanese Patent Laid-open No. 74957/1987 (which corresponds to U.S. application No. 909,472, U.S. Pat. No. 4,739,007, issued Apr. 19, 1988).
This composite material is produced by three steps of contacting, mixing, and polymerization which are explained in the following. In the contacting step, a clay mineral is brought into contact with a swelling agent in a dispersion medium such as water. In this step there is obtained a complex containing the dispersion medium which is swollen by the polyamide monomer at a temperature higher than the melting point of the monomer. Subsequently, this complex is dried to remove the dispersion medium and then pulverized using a mortar or the like. The dried and pulverized complex is mixed with a polyamide monomer in the mixing step. The resulting mixture is heated to polymerize the polyamide monomer in the mixture in the polymerization step. In this way there is obtained a composite material composed of a polyamide resin and layered silicate dispersed therein.
A disadvantage of the above-mentioned process is that the contacting step requires large-scale equipment and a great deal of energy and time for drying and pulverizing. This leads to a high production cost and a low productivity. Another disadvantage is that the complex is not uniformly mixed with the polyamide monomer in the mixing step because the resulting mixture lacks flowability. This gives rise to the lack of homogeneity in the composite material which is eventually obtained.